1. Field of the Invention
The present invention relates to an image-forming device employing electron-emitting elements.
2. Related Background Art
Hitherto thin plate-type image-forming devices have been used, in which a plurality of electron-emitting elements are arranged in a plane and are counterposed to image-forming members for forming images by electron beam irradiation (a member which emits light, changes its color, become electrified, or denaturated by collision of electrons, e.g., a fluorescent material, and a resist material). FIGS. 35 and 36 show an outline of construction of a conventional electron beam display device as an example of the image-forming device. FIG. 36 shows a sectional view at A-A' in FIG. 35.
The construction of the conventional electron-beam display device shown in FIGS. 35 and 36 is described below in detail. A rear plate 101, an external frame 111, and a face plate 109 constitute an envelope. The interior of the envelope is maintained in vacuum. Electrodes 103a and 103b, and an electron-emitting section 104 constitute an electron-emitting element 105. A scanning electrode 102a and an information signal electrode 102b are wiring electrodes, and are connected respectively to the electrodes 103a and 103b. A glass substrate 106, a transparent electrode 107, and a fluorescent material (an image-forming member) 108 constitute the face plate 109. The numeral 112 indicates a luminescent spot, and the numeral 110 indicates a supporting member for supporting the envelope against the atmospheric pressure. The electron-beam display device displays an image by application of signal voltages between scanning electrodes 102a and information signal electrodes 102b arranged in an X-Y matrix to project an electron beam onto the fluorescent material 108 in correspondence with information signals. As the electron-emitting element 105, useful are thermoelectron-emitting elements in which electrons are emitted from the electron-emitting section 104 on heating; field emission elements disclosed in U.S. Pat. No. 3,755,704 and U.S. Pat. No. 4,904,895; and surface conduction type emitting elements disclosed in U.S. Pat. No. 5,066,883.
In the above-described plane type electron beam display device, the inside of the envelope is kept at a vacuum. A supporting member 110 is provided between the rear plate 101 and the face plate 109 as shown in FIG. 36 to support the envelope internally against the external atmospheric pressure. The supporting member 110 is usually made of an insulating material to give dielectric strength against high voltage applied between the fluorescent material 108 (or the transparent electrode 107) and the electron-emitting element 105. The supporting member is indispensable for simplification, miniaturization, and weight reduction of the entire device, since an electron beam display device having a larger display surface is subjected to a larger total atmospheric pressure.
However, conventional electron beam display devices mentioned above, as shown in FIGS. 35 and 36, have a supporting member 110 made of an insulating material, which will be electrified at the surface by undesired collision of electrons and ions thereto. The electrification of the supporting member causes the problems as below: (1) The electron beam is deflected owing to the electrification, whereby the quantity of irradiation of electron beam onto the desired fluorescent material in the picture elements fluctuates to cause irregularity in luminance and color. In particular, when the quantity of the electrification is large, the electron beam is not projected to the desired fluorescent material but is directed to undesired adjacent fluorescent material to cause crosstalk; (2) The quantity of electrification varies with lapse of time, which causes time-variation of the electron path, resulting in variation in the intensity of the luminance; and (3) Electric discharge occurs at the electrified supporting member, which may damage the electron-emitting element or deteriorate the insulating property of the supporting member.
For preventing the above electrification of the supporting member by the electron beam, for example, the insulating material portion of the supporting member 110 is surrounded with a metal cover 113 as shown in FIG. 37 (sectional view)(Japanese Patent Application Laid-Open No. 64-41150). In FIG. 37, the metal cover 113 is fixed by a member 114 at the supporting member 110. The metal cover 113 is connected electrically to a transparent electrode 107. Thereby, the metal cover 113 is kept at the same voltage as the transparent electrode (fluorescent material 108). Generally, the transparent electrode 107 is kept at a high potential so as to capture the electron beam. When the metal cover is kept at a high potential is placed in proximity to the electron-emitting element 105, the electron beam emitted from the electron-emitting element 105 is deflected toward the side of the metal cover 113, causing different problems mentioned below: (4) A fraction of the electron beam is captured by the metal cover, whereby the intensity of the electron beam is lowered and the luminance of the fluorescent material is lowered at the proximity to the supporting member, causing irregularity of the luminance; and (5) The potential applied to the transparent electrode (fluorescent material) cannot exceed a certain value, whereby the luminance is low, red-light emitting and blue-light emitting fluorescent material cannot be used, and therefore a full-color image cannot be displayed.